Magneto-resistive effects include a number of different physical phenomena, all of which having in common that an electrical resistance of a resistive element is alterable by the behavior of a magnetic field penetrating the resistive element. Technologies utilizing magneto-resistive effects are sometimes referred to as “xMR technologies”, whereby the “x” indicates that a multitude of effects may be addressed here, like the Giant Magneto-Resistive (GMR) effect, the Tunnel Magneto-Resistive (TMR) effect, or the Anisotropic Magneto-Resistive (AMR) effect, to mention just a few examples. xMR effects may be applied in a variety of field-based sensors, for example for measuring revolution, angles, etc. In some applications, especially in applications relevant to safety, it is required that these sensors operate reliably and at a high level of accuracy.
Since hysteresis behavior of the sensor may result in erroneous measurement results, magnetic xMR sensor concepts with a free layer in a vortex configuration may be used. Such sensors may have nearly zero hysteresis. Low hysteresis may in other words be achieved in presence of a vortex magnetization state (magnetic field) in the free layer and may especially be interesting in applications such as wheel speed sensing, current sensing, or linear field sensing. The vortex magnetization state is characterized by having locally different orientation of the magnetization, basically pointing in all possible directions within the hemisphere. The vortex magnetization state, however, may result in formation of regions of the free layer having parallel magnetization, and such having anti-parallel magnetization with respect to a reference magnetization. Therefore, these regions may cause either low or high resistivity, respectively, to an electric current passing through them. This in turn may lead to undesirable asymmetries or non-linearity in resistance behavior as function of an external field strength. The external field and external field strength is usually the quantity to be measured. In other words, for example, changes of external field strength by identical amounts may lead to resistance shifts by different amounts, depending on whether the external field is increased or decreased in strength, or depending on a starting value of field strength. Resistance behavior in dependence of external magnetic field strength (or vortex position) may be described in the so-called resistance transfer function.
It is hence desirable to provide a sensor element exhibiting a more symmetric and more linear resistance behavior.